1. Field of Invention
This present invention relates to grazing incidence, volume slanted fringe diffraction gratings, or holograms, with high fringe contrast, and a method and system for recording and playing back the same using a single pass, grazing incidence, light beam.
2. Brief Description of Prior Art
Various types of diffraction gratings have been developed during the past half century. These gratings may have the form of a surface relief, or may be structured within the volume of a material. Volume gratings are often produced using holographic techniques. Diffraction gratings may also be characterized as being transmission type or reflection type.
While transmission and reflection type holograms have enjoyed increasing popularity over the years, increasing attention is being given to grazing incidence type holograms. Grazing incidence as used herein is intended to refer to a large or "steep" angle of incidence with respect to the normal to an interface between two surfaces. Light for reconstructing a grazing incidence hologram may enter the hologram through its edge, or at the edge of a transmissive substrate material upon which the hologram is affixed (e.g., referred to as "edge-illuminatable" or "edge-referenced"), or may achieve grazing incidence for replaying the hologram by other means. The primary reason for the increasing interest in grazing incidence holograms is their compact geometry that lends itself to convenient use in diverse applications.
Consequently, the primary goal in many holographic recording techniques is to produce grazing incidence holograms having a very thin packaging or an ultra-compact overall geometry. However, when recording grazing incidence holograms, several assumptions about usual recording constraints do not apply, because the geometry imposed on the recording substrate and reference beam during the recording process, inherently imposes major restrictions on the properties of the recording materials used. Moreover, unless such restrictions are satisfied, low contrast fringes, and thus lower diffraction efficiency will result, since it is not possible to achieve high intensity transmission of light from the reference light source, through the recording substrate and into the recording medium, where the reference beam interferes with the object beam to create the desired interference pattern (i.e., fringes).
Some of the issues associated with edge-lit holograms are presented in a paper entitled "Edge-illuminated Holograms" by Juris Upatnieks published in Volume 31, No. 8, Applied Optics, Mar. 10, 1992. In this paper the author states that the use of thin substrates and extremely steep reference beam angles is impractical. Upatnieks proposes the use of an intermediate coupling layer (e.g. index matching fluid) between the recording emulsion and the substrate, which he calls a "cover plate" and recognizes the need to perform some matching of the index of refraction of the coupling layer to both the the recording substrate and the holographic recording medium. Index matching to reduce moire effects, and extra reflections, which result in stray fringe formation are commonly used in holography. However, in the Upatnieks article, the index of refraction of the holographic recording medium is not closely matched to the index of refraction of the substrate, and this paper does not teach the degree of index matching that is required. Consequently, the edge illuminated hologram described in the other prior art is not displayable with a sufficient degree of brightness. This has thus far severely limited the practical applications for grazing incidence holograms.
There is thus a need in the art for an improved system and method of producing holograms with high fringe contrast, using very steep reference beam angles and/or very thin substrates, thereby enabling ultra-compact geometries while avoiding the shortcomings and drawbacks of prior art holographic recording systems and methodologies.